Engine knock is caused by spontaneous combustion of an air/fuel mixture in an engine cylinder (e.g., combustion chamber) outside of a combustion front from an ignition event (e.g., from a spark plug). As an example, carbon buildup within the cylinder may increase a compression ratio of the cylinder, resulting in an increased propensity for knock. A knock sensor may be utilized to identify knock so that an engine controller may employ various strategies to mitigate knock, such as by adjusting (e.g., retarding) a spark timing. The knock sensor may be a passive piezoelectric device that outputs a voltage in response to a received acoustic vibration, for example. A high amplitude knock sensor output in a knock frequency band may indicate a knock event. However, since strategies to reduce engine knock may degrade engine performance, decrease fuel economy, and increase emissions, a rationality check of the knock sensor may be used to ensure that knock reduction strategies are not employed in the absence of a knock event, such as due to false knock detection by the knock sensor.
Other attempts to address a knock sensor rationality check include determining knock sensor degradation based on the sensor output during engine operation. One example approach is shown by Hernandez et al. in U.S. Pat. No. 7,222,607 B2. Therein, a knock energy of a knock sensor is computed, and a knock sensor fault is determined if the knock energy is lower than an experimentally determined threshold.
However, the inventors herein have recognized potential issues with such systems. As one example, rationalizing the knock sensor using only output from the knock sensor itself may have low diagnostic fidelity. For example, under certain conditions, the energy of the knock sensor output may be lower than a threshold even when the knock sensor is not degraded. As an example, during a vehicle idle stop or during an electric mode of operation of a hybrid electric vehicle (HEV), the knock sensor output may be at or near zero. Under these conditions, prior approaches may lead to a false diagnosis of knock sensor degradation. As another example, comparing the knock energy to the experimentally determined threshold may not distinguish a true knock event from a falsely detected knock event, as the degraded knock sensor may have a knock energy that is higher than the threshold even in the absence of knock.
In one example, the issues described above may be addressed by a method, comprising: responsive to an indication of a knock event from a knock sensor coupled to an engine propelling a vehicle, correlating output of the knock sensor with output of a microphone on-board the vehicle to determine degradation of the knock sensor. In this way, the knock sensor may be rationalized against another on-board sensor, increasing diagnostic fidelity.
As one example, correlating the output of the knock sensor with the output of the microphone includes generating a frequency response of each of the knock sensor output and the microphone output during the knock event. The knock sensor is determined to be rational in response to a dominant frequency of the knock sensor frequency response matching a dominant frequency of the microphone frequency response, and the knock sensor is determined to be degraded in response to the dominant frequency of the knock sensor frequency response not matching the dominant frequency of the microphone frequency response. As a result, a detected knock event may be confirmed using a second on-board sensor, thereby indicating that the detected knock event is a true knock event and the knock sensor is rational. If the detected knock event is not confirmed, it may be determined that the knock sensor is degraded and detecting knock even in the absence of a true knock event. In this way, knock reduction strategies, including spark retard, are not employed in the absence of a true knock event detected by a rational knock sensor, preventing degraded engine performance, decreased fuel economy, and increased emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.